The present invention relates to a printhead and a printing apparatus using the printhead, and more particularly, to a printhead for performing printing in accordance with an ink-jet method and a printing apparatus using the printhead.
In printers using an ink-jet printhead to print an image by discharging ink, to perform high-quality printing, it is desirable to uniformly maintain the size of ink droplets, since there might be degradation of image quality due to ink droplets in non-uniformed sizes or due to density unevenness caused by such ink droplets.
Among the ink-jet printheads, in a printhead which heats ink so as to form a bubble and discharge an ink droplet by the pressure of the bubble, the size of ink droplet is influenced by the ink viscosity and the pressure upon bubble formation. As the ink viscosity and the bubble pressure depend on the temperature of the ink, when the temperature changes, the size of ink droplet also changes. As a result, the quality of print image may be degraded.
Accordingly, conventional print control is detecting the ink temperature, changing a pulsewidth to be applied to the heater so as to control discharge energy, thus maintaining a predetermined size of ink droplets. Further, another control is detecting an abnormal temperature rise which may damage the printhead during a high-speed printing operation, and suspending the printing operation.
Japanese Patent Laid-open Publication No. 6-336071 proposes temperature detection means for such control. The temperature of printhead is detected by amplifying a forward voltage (VF) from a diode integrated on the substrate of the printhead by analog signal processing outside the printhead, then performing correction on fluctuations in the forward voltage (VF), and reading a Vf temperature coefficient.
FIG. 6 is a block diagram showing the construction of a conventional temperature detection circuit capable of correcting fluctuations in output from a diode sensor.
As shown in FIG. 6, a voltage applied to a diode 102 on a substrate of a printhead 101 from a constant-voltage power supply 103 is inputted as a forward voltage (VF) into a (xe2x88x92) terminal of a differential amplifier 104 constructing an analog signal processing circuit outside the printhead. The output from the differential amplifier 104 is inputted into a (+) terminal of a comparator 105. On the other hand, a signal indicating a predetermined threshold value, outputted from a CPU 106, and converted by a D/A converter 108 into an analog signal, is inputted into a (xe2x88x92) terminal of the comparator 105. The output from the comparator 105 is inputted into an A/D input terminal 109 of the CPU 106.
On the other hand, as the CPU 106 includes an A/D converter, the analog signal inputted into the A/D input terminal 109 is converted into a digital value and is processed by the CPU 106. Further, as the printing apparatus has a temperature sensor 107 for detecting its internal temperature, the output from the sensor is inputted into an A/D input terminal 110.
In this construction, the CPU 106 performs print control based on the temperature information detected by the diode in the printhead 101.
However, in the above conventional art, as the temperature is detected by the signal processing circuit outside the printhead by using the forward voltage (VF) from the diode on the substrate of the printhead, there are following problems to be solved.
(1) The diode of the printhead and the analog signal processing circuit are electrically connected to each other via a flexible cable and connectors, and the same flexible cable includes signal and power source lines for driving the printhead. Digital noise from the adjacent signal line and noise from the power source line may mix with the signal from the diode, producing an error in the forward voltage (VF) to cause error in the detected temperature.
(2) As a heater driver for the printhead is also integrated on the semiconductor substrate where the diode is integrated, the fluctuations in a heater drive current become ground potential fluctuations due to the impedance of the ground of the substrate. Accordingly, the ground potential of the diode on the same substrate fluctuates. If the ground potential of the temperature detection circuit is different from that of the substrate of the printhead, there may be an error in the detection of the forward voltage (VF) as in the case of the above-described noise.
(3) The variations in characteristic of diodes of respective printheads and the variations in characteristic of the analog signal processing circuits also cause a similar error.
Upon occurrence of such detection temperature error, ink discharge control in accordance with ink temperature cannot be performed, and as a result, the quality of print image is degraded.
On the other hand, as the conventional temperature detection, an analog voltage as a temperature change of the forward voltage (VF) is inputted into an A/D converter of a control circuit of the printer main body and is converted to a digital value, and the temperature is calculated from the change amount, based on a table showing the relationship between a temperature and a change amount pre-stored in a ROM of the control circuit. For this purpose, the A/D converter that must be provided in the control circuit and the analog processing circuit that must be provided outside the printhead complicates the apparatus construction, and increase the costs of the apparatus.
Accordingly, it is an object of the present invention to provide a printhead, having a temperature detection circuit on its substrate, capable of temperature detection at a lower cost and with higher accuracy, and a printing apparatus using the printhead.
According to one aspect of the present invention, the foregoing object is attained by providing a printhead comprising: a driving circuit that drives printing elements; and a detection circuit that detects a temperature and outputs information on the temperature as digital output, wherein the driving circuit and the detection circuit are integrated on the same semiconductor substrate, wherein the detection circuit includes: a first circuit that generates a temperature-independent bandgap voltage; a second circuit that amplifies the bandgap voltage generated by the first circuit and generates a reference voltage; a third circuit that generates a thermal voltage proportional to an absolute temperature; and a fourth circuit that compares the thermal voltage with the reference voltage, and outputs the result of comparison as binary data.
In the printhead, preferably, the second circuit includes plural resistors serially connected to an output of the reference voltage so as to output plural different voltages among the plural resistors. Preferably, the plural different voltages are sequentially selected and outputted to the fourth circuit.
Further, preferably, the printhead further comprising: a fifth circuit that corrects the reference voltage and the thermal voltage; and holding means for holding information corrected by the fifth circuit. For example, the holding means includes a fuse ROM.
Note that it is preferable that the printhead is an ink-jet printhead that performs printing by discharging ink. In this case, the printhead has electrothermal transducers to generate thermal energy to be supplied to the ink for discharging the ink by utilizing the thermal energy.
According to another aspect of the present invention, the foregoing object is attained by providing a printing apparatus which performs printing by using the printhead having the above construction.
In accordance with the present invention as described above, the temperature detection circuit is integrated on the substrate where the printhead driver is integrated, and the temperature information is outputted as digital data from the detection circuit. More specifically, a bandgap voltage independent of temperature is generated, amplified and utilized as a reference voltage, and a thermal voltage proportional to a absolute temperature is generated. The thermal voltage and the reference voltage are compared with each other, and the result of comparison is outputted as digital binary data.
The invention is particularly advantageous since the detection circuit to detect the temperature and output the temperature information as digital data is provided on the substrate where the printhead driver is provided. In this construction, it is not necessary to provide an analog signal processing circuit outside the printhead as in the conventional art. Thus, the circuit construction for temperature detection can be simplified and provided at a lower cost.
The detection circuit has a circuit to compare the reference voltage generated based on the temperature-independent bandgap voltage and the thermal voltage proportional to the absolute temperature obtained by using a current generated in the circuit to generate the bandgap voltage, and outputs the result of comparison as a binary digital value. As a result, as the temperature information processing circuit and the temperature detection circuit are provided on the same substrate, the construction can contribute to reducing the detection temperature error due to fluctuations in the substrate potential derived from the operation of the driver upon printing operation. Further, as the temperature information is outputted as digital information, noise tolerance against noise caused by wiring or the like can be increased.
Further, the detection error due to variations in device characteristic and circuit characteristic can be minimized by providing a correction circuit.
Further, as the printhead holds the correction information, the printing apparatus using the printhead omits correction processing.
In this manner, the temperature detection can be performed with higher accuracy, and print control can be performed more appropriately based on the temperature.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same name or similar parts throughout the figures thereof.